Grid-powered AC-to-DC power converters are ubiquitous in the modern home and office. These range from low powered “wall cubes” configured to convert AC from the national electric grid through a wall socket to power cell phones and tablet computers through larger power supplies configured to power notebook and laptop computers, to the larger power supplies of desktop computers and printers.
While single-voltage power supplies, operable over a range of input voltages from 105-135 VAC, may suffice for use in the United States alone, almost all international business travelers, and most leisure travelers, carry with them one or more cell phones, electronic cameras, electric razors, tablet or laptop computers, and other portable electronic devices. Further, many such devices are manufactured for the international market, with only a plug adapter or cord and configured language selections of software differing between versions sold in multiple countries. Since many countries of the Caribbean, Europe, Asia, New Zealand, and Australia distribute grid power in the 200-250 VAC range, it has become traditional to provide multivoltage power supplies with many audio players, cell phones, tablets, and laptop computers, these are typically operable from a low voltage in the 90-100 VAC range to a high voltage in the 240-250 VAC range, and operable at either the 50 or 60 hertz frequency common among US and foreign national power grids. Many of these portable electronic devices have batteries and battery chargers in them and operate internally primarily from low DC voltages between one and twenty-four volts.
Many grid-powered AC-to-DC power converters use direct-rectification of their AC input to provide an internal high-voltage DC bus, with an internal DC-DC downconverter adapted to convert the internal high-voltage DC to a low-voltage DC power-supply output connectable to the portable electronic device. Low voltage DC outputs of 5 volts, 12-14 volts, and 18 volts are particularly common, although other output voltages are known.
Such supplies often have an architecture depicted in FIG. 1. An AC supply couples to a rectifier 102 that provides a pulsating DC current. The rectifier feeds high voltage filter 104, having at least one capacitor, that provides high voltage energy to a regulated DC-DC converter 106, in some embodiments the DC-DC converter is a buck-type converter, although other converter types may be used. In isolated power supplies, the DC-DC converter may incorporate a transformer instead of the simple inductor of a buck-type converter. The DC-DC converter has a second filter 108 that provides DC power to a load (not shown).
In typical power supplies, the high voltage filter has one or more capacitors connected across high voltage DC. This capacitor must have a high voltage rating to prevent failure since the high voltage is essentially equal to the peak voltage input to the system; nominal voltage at the capacitor may reach 350 volts when the adapter is operated from 250 VAC in Europe. Unfortunately, 350 volt capacitors are expensive and can be leaky compared to lower-voltage capacitors.